Field of the Invention
The present invention relates to an optical lens assembly and more particularly to a lens assembly for portable devices
Description of the Prior Art
In recent years, the development of the optical lens assembly in digital cameras, web cameras and portable phone cameras becomes more rapid with the sharp increase in demand of people for high image capturing quality. High resolution, large field of view for sub camera, smaller size and etc are all considered more and more by consumers
Generally, a 5 mega-pixel lens assembly is used to be a three-lens structure. And now in order to obtain an 8 mega-pixel lens assembly we usually start project with choosing a four-lens structure, which could maintain a high yield of lens assembly when trying to pursue good imaging quality. For evaluating imaging quality, relative illumination (RI) is one of the key parameters we should take in consideration since it can indicate the imaging uniformity. And in practice, solutions like reducing density of off-axis rays, shrinking vignetting may be done to refine RI performance. These solutions could be conducted by adjusting aspheric coefficients of the lens.
How to achieve good imaging uniformity, and balance the weight between high imaging quality and manufacturing feasibility is the key topics in lens assembly fabrication, therefore, there is need to construct a favorable lens assembly structure with a good adjustment of its structure and aspheric coefficients.
A U.S. Pat. No. 7,274,518B1 discloses a lens assembly, which comprises four-lens elements and an aperture stop located between the object-side of the optical system and the second lens element for controlling the brightness of the optical system. This invention provided a method of optical system with less glass, more plastic non-spherical lens to obtain a compact system. But the invention is not conducted through adjusting relationships between aspheric coefficients to gain a better RI performance.